Cement paste properties

Concrete is a composite material made of aggregates and the reaction product of the cement and the mixing water, i. e. the porous cement paste. The structure and composition of the cement paste determines the durability and the longterm performance of concrete. Concrete is normally reinforced with steel bars. The protection that concrete provides to the embedded steel and, more in general, its ability to withstand various types of degradation, also depends on its structure. This chapter illustrates the properties of the most utiHsed cements and the microstructure of hydrated cement pastes. Properties of concrete and its manufacturing are discussed in Chapter 12. 

On the basis of the results reported in Fig. 3 it seems that the contemporary use of limestone powder and superplasticizer, in order to improve cement paste properties by lowering its porosity, allows to recover the loss of strength due to the use of 10% rubber waste particles of whatever type. This result is further confirmed by data reported in Table 3. 

Concrete is a particulate composite of stone and sand, held together by an adhesive. The adhesive is usually a cement paste (used also as an adhesive to join bricks or stones), but asphalt or even polymers can be used to give special concretes. In this chapter we examine three cement pastes the primitive pozzolana the widespread Portland cement and the newer, and somewhat discredited, high-alumina cement. And we consider the properties of the principal cement-based composite, concrete. The chemistry will be unfamiliar, but it is not difficult. The properties are exactly those expected of a ceramic containing a high density of flaws. 

The properties of these cements - the fluidity of the mix, the working and setting times of the cement paste, and the strength of the set cement - are affected by a number of factors. These include the composition of the powder, the concentration, molecular mass and type of the polyacid, the... 

Consistency, working time, setting time and hardening of an AB cement can be assessed only imperfectly in the laboratory. These properties are important to the clinician but are very difficult to define in terms of laboratory tests. The consistency or workability of a cement paste relates to internal forces of cohesion, represented by the yield stress, rather than to viscosity, since cements behave as plastic bodies and not as Newtonian liquids. The optimum stiffness or consistency required of a cement paste depends upon its application. 

The essential step is the efficient grinding and blending of raw materials. The final properties of cement strongly depend on its mineral composition so that raw composition and firing conditions are adjusted, depending on the type of cement to be produced. The microstructure of the steel fiber-cement paste interface was studied by scanning electron microscopy (SEM). The interfacial zone surrounding the fiber was found to be substantially different from the bulk paste further away from the fiber surface. The interfacial zone consisted of... 

The most important characteristic of cement is its pore structure and aqueous phase hence, the microstructure of the hardened cement paste via the pore system. It is highly alkaline (pH >13) due to rapid and almost quantitative dissolution of Na and K salts from the cement clinker. The porosity of the paste comprises interconnected and isolated pores, the pore sizes of which are important to the strength and dimensional stability of cement products. Different types of cement are used to meet different performance criteria. Properties can be estimated from compositions and fineness (i.e., particle size and size distribution). In the past, additives... 

It is known that some of the properties of fresh concrete can be considered in terms of the rheological properties of the cement paste contained in the concrete. Thus a high water-cement ratio concrete will contain a paste content which is more fluid than that of a low water-cement ratio concrete. 

In order to understand the way in which superplasticizers affect the properties of concrete, studies have often been made on cement pastes in view of the convenience of such investigations. 

Normal concrete mixes resist being moved under pressure due to segregation of the cement paste that occurs when pressure is applied. Basically a pumpable concrete will have a suitable aggregate-void system and a cement paste consistency that will flow adequately through the void channels. Both these properties of the mix are required to meet the primary demands of pumpability which are ... 

Supercritical C02 treatment affects the microstructure of the cement paste. In the first stage of the sc C02 treatment, free water in the cement pores is extracted. As a consequence of this dehydration process, channels of about 50-pm diameter develop. Dissolved calcium in the free water reacts with the C02 and crystallizes with the C02 as calcite along the channel walls. In the second stage, the structural water of the hydrated cement phases is extracted. The carbonation of the portlandite to form more calcite takes place. Water, bound to the CSH surrounding the partially hydrated cement clinker particles, is partially replaced by a carbonate formation. The short fibers of the CSH-cement framework, which are responsible for the physical properties of the cement, are not affected (Hartmann et al., 1999). 

Neubauer, C.M. (1997) On the Chemistry, Micro structure, and Deformation Properties of Cement Pastes Towards a New Strategy for Controlling Drying Shrinkage. PhD. Thesis, Northwestern University. 

Previous investigations of these hydration reactions at room temperature have been reviewed recently (4). Research in this laboratory has included the stoichiometry of the hydration of both silicates, employing different methods of hydration (2, 3, 5, 21), and a determination of the surface energy of tobermorite, the calcium silicate hydrate produced in the hydration of both silicates under most experimental conditions (8). The surface area and the surface energy of tobermorite are briefly discussed by Brunauer (I). These properties play vital roles in determining the strength, dimensional stability, and other important engineering properties of hardened portland cement paste, concrete, and mortar. 

Structure and properties of fresh and hardened Portland cement pastes... 

The workability of a concrete mix is by no means dependent only on the physical properties of the cement paste it contains, but an understanding of it requires one of those properties. For some specialist uses in which cement is used without an aggregate, the latter are directly relevant. The most important properties are concerned with rheology, and this section deals primarily with these properties in Portland cement pastes, free from admixtures, prior to setting. From the chemical standpoint, this period comprises that of initial reaction and induction period. From the practical standpoint, it includes those of mixing, placing and compaction. 

---